Abstract
Anodic oxidation of various aluminium alloys was investigated by means of rotating disc electrodes in 3 M H2SO4 as a function of Cl−, F−, Zn2+ and In3+ concentration. Al-In, Al-Zn/In and Al-Zn/Sn alloys yielded current-potential curves at the lowest overpotentials and faradaic efficiencies for anodic oxidation of up to 98% at currents ⩾ 50 mA cm−2. While these alloys were electrochemically active in the presence of chloride as the only additive in sulphuric acid, binary aluminium alloys with Ce, Ga, La, Nd, Sn, Ta, Te, Ti or Tl were only active when Cl−, Zn2+ and In3+ species were added to the electrolyte. With the exception of Al-Ga, binary alloys displayed high faradaic efficiencies of up to 95%. Fluoride additives resulted in current-potential curves at even more negative potentials than those with chlorides. In contrast to Cl−, fluoride ions are consumed during the aluminium oxidation process due to complexation with Al(III).
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